With a compact disk (CD) or a digital versatile disk (DVD), it is common that recording of binary signals and detection of tracking signals are carried out by utilizing a change in reflectivity caused by interference or reflected lights from the mirror surface and the bottom of pits.
In recent years, phase-change type rewritable compact disks (CD-RW, CD-Rewritable) or phase-change type rewritable DVD (tradename: DVD-RW, DVD+RW, in this specification, rewritable DVD may sometimes be referred to as RW-DVD) have been used as optical recording media interchangeable with CD or DVD.
Such phase-change type CD-RW or RW-DVD utilizes a phase difference and a difference in reflectivity caused by a difference in the refractive index between an amorphous state and a crystalline state to detect recording information signals. A usual phase-change type CD-RW or RW-DVD has a structure comprising a substrate, and a lower protective layer, a phase-change type recording layer, an upper protective layer and a reflective layer, formed on the substrate, so that multiple interference of these layers can be utilized to control the difference in reflectivity and the phase difference and to provide interchangeability with CD or DVD. Further, recording on CD-RW or RW-DVD means recording by overwriting wherein recording and erasing are carried out simultaneously.
As a result, although it is difficult to secure interchangeability covering a high reflectivity as high as at least 70%, it is possible to secure interchangeability of recording signals and groove signals within a range where the reflectivity is lowered to a level of from 15 to 25% in the case of CD-RW or to a level of from 18 to 30% in the case of RW-DVD, and retrieving can be carried out by a current CD drive or DVD drive for retrieving only, if an amplifying system to complement the low reflectivity is added to the retrieving system.
However, one of problems in using CD-RW or RW-DVD is that the recording velocity and the transfer rate are low. The reference velocity (hereinafter referred to also as 1-time velocity) in recording/retrieving of CD is a linear velocity (in this specification, “a linear velocity” may simply be referred to as “linear speed” of from 1.2 to 1.4 m/s. However, for CD-ROM, a high velocity retrieving at a level of 40-times velocity at the maximum has been already realized, and a low velocity at a level of 1-time velocity is used only for retrieving of musics or images. Usually, in up to 16-times velocity retrieving, a constant linear velocity mode (CLV) inherent to CD is used, but in 24 to 40-times velocity retrieving, the transfer rate, access and seek times for the outer periphery data have been remarkably speeded up by an application of a constant angular velocity mode (CAV).
Speeding up in recording is in progress also for CD-RW, but in CLV mode, the speed is still at a level of 12-times velocity at best. Usually, with CD-RW, it takes a time as much as 74 minutes (or 63 minutes) if recording is made over the entire surface at 1-time velocity, and even at 12-times velocity, it takes about 6 minutes. However, at 20-times velocity, recording can be completed in 5 minutes, whereby the application of CD-RW can be substantially broadened for recording data of large amounts such as musics and images.
Further, as a peripheral memory device for a computer, CD-R has already accomplished 24-times velocity for recording, and also for CD-RW, it is desired to increase the transfer rate in recording.
On the other hand, the reference velocity (hereinafter referred to also as 1-time velocity) in retrieving of DVD is a linear velocity of 3.49 m/s, but with DVD-ROM, high velocity retrieving at a level of 16-times velocity at the maximum has already been realized, and a low velocity at a level of 1-time velocity is used only for retrieving of musics or images.
Speeding up in recording is in progress also for RW-DVD, but in CLV mode, it is still at a level of 2.4-times velocity at best. Usually, with RW-DVD, it takes a time as much as about 60 minutes if recording is carried out over the entire surface at 1-time velocity, and even at 2.4-times velocity, it takes about 25 minutes. However, at 6-times velocity, recording can be completed in 10 minutes, and application of RW-DVD can be substantially broadened for recording data of large amounts such as musics or images.
Therefore, a phase-change medium and a recording method have been desired whereby recording can be carried out at a higher velocity.
However, a rewritable phase-change medium capable of recording up to a high linear velocity of at least 20-times velocity for CD or at least 6-times velocity for RW-DVD, has not yet been realized. This means that a rewritable CD or DVD medium which is overwritable at a high linear velocity at a level of exceeding a linear velocity of 20 m/s, has not yet been realized.
A first reason for why such a rewritable phase-change medium can not be realized, is that it is difficult to simultaneously satisfy the archival stability of amorphous marks and erasing in a short time by high speed crystallization of amorphous marks.
For example, with a recording material comprising a SbTe alloy as the main component which is used as a material for a recording layer of CD-RW overwritable at 1 to 4-times velocity or RW-DVD overwritable at up to about 2.4-times velocity, high speed crystallization is possible by relatively increasing the Sb content, whereby overwriting at a linear speed of at least 20 m/s will be possible. However, according to a study made by the present inventors, it has been found that such an increase of the Sb content tends to substantially impair the archival stability of amorphous marks, whereby amorphous marks will disappear by recrystallization to such an extent that no retrieving is possible, within 1 to 2 years at room temperature or in a few days in a high temperature environment at a level of from 50 to 80° C. in the interior of the recording apparatus. Otherwise, there is a serious problem such that amorphous marks start to disappear by repeated retrieving from about a few hundreds to a few thousands times by a laser beam of at most 1 mW, and it has been found that the reliability as a recording medium can not be maintained.
In addition to the necessity to solve such a problem, CD-RW or RW-DVD has a restriction such that it is necessary to secure retrieving interchangeability with a widely used CD-ROM drive or DVD-ROM drive for retrieving only.
For example, in the case of CD-RW, in order to secure retrieving interchangeability, it is necessary to satisfy not only a high modulation at a level of a modulation of from 55 to 70% but also a reflectivity of from 15 to 25% and other servo signal characteristics. On the other hand, in the case of RW-DVD, in order to secure retrieving interchangeability, it is necessary to satisfy not only a high modulation at a level of a modulation of from 55 to 70% but also a reflectivity of from 18 to 30% and other servo signal characteristics.
Further, a second reason for why CD-RW or RW-DVD overwritable at a high linear velocity of at least 24 m/s has not yet been realized, is that a fairly strict recording pulse strategy (pulse division method) is specified in CD-RW standards or RW-DVD standards.
Namely, in CD-RW standards Orange Book, Part 3, a recording pulse strategy as shown in FIG. 1, is specified. Accordingly, in a currently practically used recording device, IC for generating such a recording pulse strategy is employed. Accordingly, with a currently practically used recording device, it is obliged to carry out recording in a wide range of linear velocity ranging from 1-time velocity to 8- to 10-times velocity by such a recording pulse strategy or by a recording pulse strategy having certain changes made thereto.
Also in standards for DVD-RW or DVD+RW as standards for rewritable DVD, a similar recording strategy is specified. A characteristic of such a recording strategy is that an amorphous mark having a nT mark length is divided into n−1 recording pulses and cooling pulses (off-pulses) for recording. Therefore, in such a recording strategy, an average repeating period for a pair of a recording pulse and a cooling pulse is made to be about 1T.
FIG. 1(a) shows EFM modulated data signals having time lengths of from 3T to 11T, and FIG. 1(b) shows the practical recording laser powers generated on the basis of such data signals. Pw represents a writing power to form an amorphous mark by melting and quenching the recording layer, Pe represents an erasing power to erase an amorphous mark by crystallization, and usually, a bias power Pb is substantially the same as a retrieving power Pr of a retrieving laser beam. Writing power (Pw) irradiation sections are referred to as recording pulses, and bias power irradiation sections are referred to as off-pulses.
In the case of EFM+ modulation, data signals having time lengths of 14T are added to the above-mentioned data signals having time lengths of from 3 to 11T.
Here, in the above-mentioned recording strategies, a repeating period for a recording pulse and an off-pulse is basically constant as a reference clock period T. The reference clock period T is made to have a high frequency in proportion to the linear velocity in high linear velocity recording.
At a reference velocity of 1-time velocity for CD, T=231 nsec, but at 24-times velocity, T=9.6 nsec, and at 32-times velocity, T=7.2 nsec. Accordingly, in a case where the recording pulse strategy shown in FIG. 1 is used in high linear velocity recording at least 24-times velocity, the time widths of divided recording pulses and off-pulses in FIG. 1 will be less than 5 nsec by the above-mentioned change for high frequency corresponding to the high velocity recording.
On the other hand, at a reference velocity of 1-time velocity for DVD, T=38.2 nsec, but at 6-times velocity, T=6.4 nsec, and at 8-times velocity, T=4.8 nsec. Accordingly, in high linear velocity recording at least 6-times velocity, the time widths of divided recording pulses and off-pulses in FIG. 1 will be at most 3 nsec by the above-mentioned change for high frequency corresponding to such high velocity recording.
Whereas, by irradiation with a laser beam having a usual writing power, it takes from 1 to 3 nsec in rising or falling. Accordingly, at such a high frequency, the rise time or the fall time can not be neglected, and the lengths of recording pulse sections and the lengths of off-pulse sections will further substantially be shortened and will be substantially less than 5 nsec (in the case of CD-RW) or less than 3 nsec (in the case of RW-DVD). In such a case, heating for recording pulses tends to be inadequate, and the required writing power will be sharply high. On the other hand, cooling for the off-pulse sections also tends to be inadequate, whereby a cooling rate required for the change into an amorphous state tends to be hardly obtainable. Further, for the high linear velocity recording, it is common to employ a material having a high erasing speed i.e. a high crystallization speed for the recording layer for CD-RW or RW-DVD. Accordingly, deficiency in the cooling rate for the above-mentioned off-pulse sections, tends to lead to recrystallization of the once-melted region.
Accordingly, with the recording pulse strategy shown in FIG. 1, it is very difficult to carry out high velocity recording at a level of at least 24-times velocity on CD-RW or to carry out high velocity recording at a level of at least 6-times velocity on RW-DVD.
In order to solve such problems, some of the present inventors have already realized overwriting on CD at 16-times velocity or on DVD at 5-times velocity by a division method wherein the repeating period of a recording pulse and an off-pulse is set to be 2T base (JP-A-2001-331936). However, even if such a division method of 2T base is employed, it is necessary, as mentioned above, to employ a material having a high crystallization speed for high linear velocity recording at a level of at least 24-times velocity for CD or at a level of at least 6-times velocity for DVD, while, if such a material is employed, the recrystallization phenomenon will be more serious due to deficiency of the cooling rate.
It is an object of the present invention to provide a rewritable optical recording medium to be used for high velocity recording at a level of at least 20 m/s and a recording method therefor.
A specific object of the present invention is to provide CD-RW to be used for high velocity recording at a level of at least 24-times velocity and a recording method therefor. More specifically, the object is to provide a rewritable medium having retrieving interchangeability with CD with respect to the recording signal format and a recording method therefor, wherein in CD-RW, an amorphous state of the recording layer corresponds to a recorded mark, and mark length recording is carried out by EFM modulation (i.e. by a combination of mark lengths and space lengths between marks, having time lengths of from 3T to 11T, based on the reference clock period T of data).
A specific object of the present invention is to provide a rewritable DVD recording medium to be used for high velocity recording at a level of at least 6-times velocity. More specifically, the object is to provide a rewritable medium having retrieving interchangeability with DVD with respect to the recording signal format, and a recording method therefor, wherein an amorphous state of the recording layer corresponds to a recorded mark, and mark length recording is carried out by EFM+ modulation, i.e. a combination of mark lengths and space lengths between marks, having time lengths of from 3T to 14T, based on the reference clock period T of data.